Networks
description
Transcript of Networks
Networks
Chapter 3
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What Is The Internet?
It's not a big truck. It's a series of tubes.Ted Stevens, former Alaskan Senator, 6/28/2006
Internet: global system of interconnected computer networks
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Client/Server Architecture
Most interactions over the Internet use the client-server model.
client: application or system that accesses a remote service on another computer system, known as a server, by way of a network
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Most Common Client?
Web browsers request documents (web pages) from web servers. Examples:
Firefox Internet Explorer Safari Opera
WWW (World Wide Web): system of interlinked documents (web pages) accessible via the Internet The WWW is a subset of the Internet.
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IP Address
IP (Internet Protocol) address: numerical address given to each computer connected to the Internet An IP address consists of four numbers (ranging
from 0 to 255) separated by periods. Examples:
128.95.1.207 209.131.36.158 4.2.2.1
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Getting an IP Address
DHCP (Dynamic Host Configuration Protocol): protocol for assigning IP addresses to devices on a network
protocol: standard procedure for sending and receiving data between computers Examples:
HTTP (Hypertext Transfer Protocol) SFTP (Secure File Transfer Protocol) SMTP (Simple Mail Transfer Protocol) IMAP (Internet Message Access Protocol)
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Getting an IP Address Via DHCP1. Client broadcasts need for an IP address.2. DHCP servers on the network respond.3. Client chooses a server and sends the MAC
(Media Access Control) address of its network card.
MAC address: globally unique identifier assigned to network cards by manufacturers
4. DHCP server responds with an IP address and other useful information. The server records that the address is now in use.
DHCP servers can also refuse to give an IP address for security reasons or if there are no more free addresses.
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Domain Name System (DNS)
Unwieldy to remember IP address if you want to request information from another computer
domain name: human-readable name given to a related group of networked computers
Domain Name System (DNS): hierarchical naming system for computers connected to the Internet
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Internet Domain Hierarchy
tracer.cs.washington.edu
General
Specific
top-level domains
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Top-Level Domain top-level domain: last part of domain name
Examples: .com (commercial) .org (organization) .edu (education) .gov (government) .uk (United Kingdom) .ca (Canada)
Some top-level domains (e.g., .com and .org) are open for registration to anyone, whereas others (e.g., .gov and .edu) have rules restricting eligibility.
List of top-level domains: http://www.iana.org/domains/root/db/
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Top-Level Domain
In the second quarter of this year (2009), rules for top-level domain names will be relaxed to include any name. http://www.icann.org/en/announcements/announcement-4-
26jun08-en.htm
For example, the city of Seattle could get .seattle and Apple could get .apple or .mac
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Domain Name System
How do the convenient domain names like www.yahoo.com get translated into the IP addresses like 209.131.36.158 that computers need?
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DNS Server
DNS server: computer that keeps a list of the human-readable names and the corresponding IP addresses
How does a computer get the IP address of a DNS server? Provided by the DHCP
server
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Sending Data Over The Internet TCP/IP (Transmission Control Protocol/Internet
Protocol): protocol providing reliable delivery of data across a network http://www.cs.washington.edu/education/courses/cse100/0
9wi/lectures/packetswitch.swf
Packets take several hops to be delivered. Number of hops can change depending on traffic on the
Internet http://www.webhostingshow.com/2008/03/21/how-to-tracer
oute-on-windows-and-mac/
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Sending Data Over The Internet
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How Many Hops Does It Take?
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Uniform Resource Locator (URL) Uniform Resource Locator (URL): an
identifier that specifies where a resource is available Also referred to as a web address Examples:
http://www.washington.edu/students/reg/0809cal.html https://weblogin.washington.edu/
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Anatomy of a URL
http://www.washington.edu/students/reg/0809cal.html
pathname (or path): name of a file or directory, specifying a unique location in a file system The pathname tells the server which file (page) is
requested and where to find it.
HTTP (Hypertext Transfer Protocol): protocol for retrieving interlinked documents (web pages)
protocol server's name pathname
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File Structure
directory (or folder): collection of files, other directories, or both
directory hierarchy: directories can contain other directories, which can contain other directories, etc…
root directory: directory at the top of the hierarchy
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Directory Hierarchy
Going down the hierarchy means into subfolders Going up the hierarchy means into parent
(enclosing) folders; that is, towards the root
going down the hierarchy
going up the hierarchy
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Pathname
Pathnames follow the hierarchy to the directory or file location separating components with slashes
... /galleries/gal100/pioneer.html
In Windows, backslashes are used instead of slashes (e.g., C:\cygwin\etc\fonts\local.conf)
going up the hierarchy
going down the hierarchy
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File Structure
directory contents
root directory
directories
path to "fonts" directory
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Requesting a Web Page
1. Client requests web page from web server.2. Server responds with web page.3. Browser reads the web page and makes additional
requests for images and any other files that form a part of the web page.
Additional requests might be to other servers.
4. Servers send requested resources.
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Client Server GET www.yahoo.com
Retrieve data
GET image.gifRetrieve image
Time
Requesting a Web Page
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Web Root Directory
A web site's root directory is the topmost directory (in the directory hierarchy) accessible through a web browser.
This directory might not be the absolute topmost directory in the directory hierarchy.
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Your Web Root Directory
At the UW, your web site's root directory is called public_html (see Lab #2), but public_html is in a directory named after your UW NetID.
public_html is not the absolute topmost directory, because it is in another directory!
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URL Pathname The pathname portion of a URL is relative to the web
site's root directory.
For example, if the UW's web site's root directory is called public_html, then the file 0809cal.html in the URL:
http://www.washington.edu/students/reg/0809cal.html
is located on the server at
… public_html/students/reg/0809cal.html
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No Filename In URL?
The URL http://www.cs.washington.edu/events points to the "events" directory (i.e., it does not specify a web page)
What page does the web server return to the client in that case? The web server will look for files with pre-defined names
(like index.html or index.htm) in the specified directory in the URL.
If no file in the directory has any of the pre-defined names, then the server will respond with a "File not found" error.
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Exercise
The URL http://www.cs.washington.edu does not specify a web page. What directory does the URL point to? The web root directory of UW CSE's web space.
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Browser Tip
Instead of typing http://www.yahoo.com in the address bar, just type yahoo.com
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Connecting To The Internet From Home Internet Service Provider (ISP): company that
offers its customers access to the Internet Connection types to ISP:
Dial-up DSL (digital subscriber line) Cable Internet
broadband: generic term for "fast" Internet access, typically contrasted with dial-up access over a modem
slow
faster
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Router
router: networking device that routes and forwards information
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Router
Can be used to connect multiple computers to a single Internet connection
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Wireless Networks
access point (AP): device that allows wireless communication devices to connect to a wireless network
Most home wireless access points are also routers.
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Peer-To-Peer
In a peer-to-peer (P2P) network, peer nodes simultaneously function as both client and server to the other nodes on the network. Some P2P networks have directory servers that inform
peers of the network addresses of other peers
peer-to-peer client-server